Transmissions and other types of valve bodies typically contain several valve assemblies that move in response to hydraulic pressure. In an automobile transmission, for example, numerous valve assemblies may be in fluid communication with one another, with each valve assembly independently oscillating in response to hydraulic pressure changes occurring in the transmission. While there are many different types of valve assemblies, a typical valve assembly includes a spool, a spring, a plug, and a retaining pin. These valve assemblies reside inside a mating bore, a hole in the valve body that is sized for the corresponding valve assembly.
FIGS. 1A and 1B are schematic diagrams of typical prior art hydraulic circuits. Valve body 100 contains a fluid circuit that includes a valve assembly 104, which includes a spool 108, a spring 112, and a plug 116. Valve assembly 104 communicates with other components in the fluid circuit, such as fluid strainer 120 and receiving valve assembly 124, via a first fluid line 128 and a second fluid line 132.
FIG. 1A shows valve assembly 104 in the substantially open position inside mating bore 102. While in the open position, first fluid line 128 delivers fluid to a chamber 136, which then exits to second fluid line 132. Fluid leaving chamber 136 flows to receiving valve assembly 124 and to a balance pressure line 140. As shown in FIG. 1A, balance pressure line 140 is at least partially blocked by spool 108. In operation, as hydraulic pressure builds in second fluid line 132 (a result of diminished fluid exiting the receiving valve assembly 124 and continuing accumulation of fluid from first fluid line 128), fluid enters behind spool 108 via balance pressure line 140. As fluid accumulates behind spool 108, spool 108 moves against spring 112 to the substantially closed position shown in FIG. 1B.
While in the substantially closed position, the hydraulic pressure in second fluid line 132 decreases as fluid exits receiving valve assembly 124. As the hydraulic pressure decreases, spring 112 moves spool 108 into the substantially open position, thus restoring access to chamber 136 by first fluid line 128.
As the valve moves back and forth in response to changes in hydraulic pressure, the spool lands brush against the mating bore. The repeated oscillations wear down the spool lands, the mating bore, or both. The wear allows fluid that would otherwise be contained in a valve chambers to spread into the worn area between the spool land and the mating bore. In cases where the wear is sufficient, the fluid may move from one valve chamber to another, effectively reducing the ability of the valve to effectively control fluid communications, thus disrupting fluid control in the valve body.
Repairing a worn mating bore and valve assembly is both time consuming and costly. Typically, the mating bore must be reamed to a larger size and a new, larger valve assembly is inserted. While this operation will correct the problem, the tooling required to ream the mating bore is expensive and the repair is labor intensive.